Dear Exan,
The gap at the top of your iPhone 15 screen after replacing the battery could be due to:
Improper adhesive removal/installation
Misaligned top speaker grille
Screen frame or clips not being fully engaged
SIze/fit of the replacement parts.
Please power down your iPhone and carefully disassemble the affected area to check these potential issues. If you're uncomfortable doing this yourself, consider visiting a professional repair technician.
Best,
Christian/iFixit Translator